Acute kidney injury (AKI) is a common event in cardiac surgery, with 5-30% of patients developing clinically significant AKI. AKI is a condition in which the kidneys become unable to adequately filter the blood, causing toxic levels of waste to accumulate throughout the body. AKI is formally defined as a greater than 50% decrease in glomerular filtration rate (GFR) over a period of hours to days, which leads to a decline in urine output over time. At present, measuring urine output is one of the most acceptable forms of assessing a patient for AKI, and is an important component of the established RIFLE criteria that assess kidney function.
In current clinical practice, Foley catheters are connected to urine collection vessels that are inscribed with volumetric scales. These are used by anesthesiologists intra-operatively and in the intensive care unit (ICU) to manually observe and measure urine output at certain time intervals. However, this method is subjective, and very rarely are measurements taken at frequent enough intervals for any incremental changes to be recorded. Bulk urine volume measurements are not reliable indicators of kidney function because these do not take into account the potential effects of medications and fluids that patients receive, or individual patient histories.